This invention relates to nematic liquid crystal compositions having positive dielectric anisotropy and no visible light absorption. More particularly, it is directed to a nematic liquid crystal composition including primarily liquid crystals of a cyclohexanecarboxylic ester and an ester having an acyloxyl group. These compositions have improved time-sharing characteristics which permit satisfactory time-sharing operation with a duty ratio of up to at least 1/32 at a voltage not exceeding 10 volts.
Twisted nematic liquid crystal display devices can be driven in a static or time-sharing mode. Time-sharing driven systems require a reduced number of leads and therefore are advantageous, since they (1) simplify the construction of a liquid crystal cell, (2) reduce the number of connections between the liquid crystal cell and the output of a driving circuit, and (3) simplify the construction of the driving circuit. Large-capacity display devices require operation in a time-sharing mode at a low duty ratio as the number of leads increases.
In liquid crystal display devices operating in a time-sharing mode, the driving voltage which may be applied to the liquid crystal is limited. A driving voltage which provides satisfactory liquid crystal display contrast and angle of vision is at least about the threshold voltage for obtaining a light transmittance of 30 percent. This driving voltage may vary as a result of factors such as change in battery voltage over the useful life of the battery and variations in circuit design. Accordingly, circuits are designed to operate within a voltage margin M, which is a measure of allowable fluctuations in driving voltage.
In liquid crystal display devices, selective and half-selective waveforms have an effective voltage ratio which usually decreases with a reduction in the duty ratio. There is a decrease in the difference between the threshold voltage for light transmittance of 30 percent and 80 percent, and therefore, a reduction in the voltage margin M. If the duty ratio is even further reduced, the threshold voltage for 30 percent transmittance will become greater than that for 80 percent transmittance, and the voltage margin will become negative. Under such circumstances, the duty ratio makes operation in the time-sharing mode impossible.
Many conventional liquid crystal compositions consist mainly of an azoxy liquid crystal, which deteriorates in visible light and, in consequence, reduces the quality of the display. To overcome this problem, the display devices incorporates a yellow filter to block the harmful light rays. Thus, these displays are possible only in a yellow color and not in other colors.
Various non-azoxy liquid crystals are known which do not require a yellow filter. These liquid crystals include primarily liquid crystals compound of the following structural formulas: ##STR1## where R stands for a straight-chain alkyl group. A liquid crystal mixture composed of any of these liquid crystal compounds results in a negative voltage margin in a time-sharing operation with a duty ratio lower than 1/16. These compositions are not suitable for a large-capacity display, since it is impossible to operate in the time-sharing mode with such a duty ratio.
Accordingly, there exists a need for an improved twisted nematic liquid crystal display device having a large-capacity, high-quality display, employing a liquid crystal composition which allows a time-sharing operation having a lower duty ratio and wider voltage margin than heretofore possible.